Hmmm, not sure.
2 of my kitties appeared to have lost their voice at one time and never got it back. It wasn't a medical thing, it was a spoiled thing. My vet explained that Cuddles and Cammie didn't need to meow anymore (a mere squeak would do) because I waited on them hand and foot and they realized that they didn't need to expend the energy to get my attention. Maybe it's the same with Mason?